Force sensitive scale for fork lifts with electronically coupled load sensors

ABSTRACT

A force sensitive scale and dual sensor load cells for use therewith isolate, measure, and reject out residual rejected effects from the output of the scale. By isolating residual rejected effects, the overall accuracy of the scale may be improved, and the scale may be made substantially insensitive to the position of an object on the scale. In addition, where a scale includes multiple load cells coupled in parallel to a load receiving member and spaced apart in the direction of the force vector, the outputs of the load cells may be effectively scaled to cancel out frictional effects inherent in the scale.

This is a divisional of application Ser. No. 08/491,034 filed on Jun.16, 1995 U.S. Pat. No. 5,837,946.

FIELD OF THE INVENTION

The invention is directed generally to the area of force measurement,for example in force sensitive scales such as those used on lift trucksand the like. The invention is in particular directed to a scale and aload cell used therein which reject many non-load force effects, suchthat the scale exhibits increased accuracy and reduced sensitivity tothe position of objects on the scale.

BACKGROUND OF THE INVENTION

Force sensitive scales, i.e., scales which are capable of measuring theapplied force due to gravity of an object placed upon the scale, havenumerous applications. One such application, for example, is on lifttrucks, or forklifts, to weigh objects which are lifted and transportedby the forks of the lift truck as they are being transported. Lift truckscales preferably do not require any specific operation by an operatorof the lift truck other than positioning the truck and lifting theobjects. As a result, the weighing operation is very efficient andnon-intrusive.

One example of a lift truck scale which is particularly useful in theart is the subject of U.S. Pat. No. 4,421,186 to Bradley, which isincorporated by reference herein. Generally, Bradley discloses a lifttruck scale including a vertically oriented plate that is mounted to theexisting crossbars disposed on the front of a standard lift truck. Theplate is mounted through four load-bearing load cells arranged into twolaterally-spaced vertical columns, which serve as the sole mechanicalconnection between the scale and the existing crossbars. Theconventional forks used with the lift truck are hung off of the verticalplate in the same manner as they would the crossbars, whereby a loadpresent on the forks is transferred across the load cells by thevertical plate.

Each load cell in the Bradley design includes four strain gaugesconnected in a wheatstone bridge circuit to reject many non-load forceeffects. Moreover, the outputs of the load cells are summed to providethe total weight reading for the scale.

The Bradley lift truck scale is capable of rejecting many non-load forceeffects (i.e., forces and moments which are not found along the primaryforce vector that is being measured). The Bradley scale is thereforerelatively accurate, typically having an accuracy of about 1%. Bradleyalso has the advantage that it is easy to install on conventional lifttrucks, thereby making it relatively cost effective. Also, the Bradleyscale has a low profile which does not significantly decrease theoverall carrying capacity of the lift truck.

On the other hand, the Bradley scale has been found to exhibit somedegree of position sensitivity despite efforts to reduce the variationsthat occur as a result of different positioning of objects on the forks.These effects are most prevalent in the fore/aft direction (i.e., alongthe longitudinal axes of the forks), primarily due to "end effects"applied to the load cells at some positions. The end effects aremaximized when an object is located near the tips of the forks, as arelative large horizontal separation exists between the load cells andthe object in this position, resulting in non-load forces that arecomparable in magnitude to the load forces along the primary forcevector for the scale.

By "end effects", we mean primarily end loads and end moments, as wellas other forces, that result in forces applied to the load cells thatare not along the force vector of the desired forces to be measured(i.e., those representing the weight of an object). These end effectsare mostly rejected in scales such as Bradley by the mechanicalstructure of the load cells and the strain gauge bridge circuits.However, it has been found that, particularly in lift truck scales whereend effects are relatively great, these end and other effects are notpurely rejected. This results in "residual rejected effects" beingpresent in the output signal of each load cell, thereby limiting theaccuracy of the scale and making the scale sensitive to the position ofobjects thereon.

It has also been found that the vertical arrangement of pairs of loadscells in the Bradley scale may introduce some degree of frictionaleffects (primarily creep and hysteresis) into the outputs of the loadcells due to slippage in the mechanical couplings between the plate andthe load cells, as well as due to relaxation in the structures of theplate and the load cells alike. In theory, a pair of vertically orientedload cells coupled in parallel should perfectly share a load such thatthe summed outputs of the cells remain constant even if the load isshifted between the load cells. In practice, however, it has been foundthat these frictional effects may limit this load sharing effect andintroduce lag errors into the system, also reducing the accuracy of thescale.

Therefore, a substantial need exists for a scale and load cells for usetherewith which more perfectly reject residual rejected effects andfrictional effects from the output of the load cells. In addition, aneed exists for a scale which is of greater accuracy and reducedsensitivity to the position of objects on the scale.

SUMMARY OF THE INVENTION

The invention addresses these and other problems associated with theprior art in first providing a scale and a load cell therefor whichseparately isolate, measure and reject out residual rejected effectsfrom the output of the scale, thereby offering such advantages asincreased accuracy and load position insensitivity. The invention alsoaddresses many problems in the art by also providing a scale in whichthe outputs of multiple load cells coupled in parallel by a non-perfectload receiving member and spaced apart in the direction of the primaryforce vector are scaled in such a manner that frictional effects(typically creep and hysteresis) present in the load cells and the loadreceiving member are rejected from the output of the scale.

By "residual rejected effects", we mean any non-load forces, applied inone or more force vectors which are separate from the force vector ofthe desired force to be measured. These types of effects, if notrejected, will typically introduce errors into the outputs of the loadcells, as well as the summed output of the scale.

These types of effects are termed "rejected" because they are theeffects that are typically reduced by means of the particular structureof a load cell and the arrangement of force sensors thereupon, as wellas by electrical means such as wheatstone bridge circuits. These effectsare also termed "residual" because many of these effects will stillaffect the output of the scale to a degree even after mechanical orelectrical rejection of most of the effects. However, it will beappreciated that it is not necessary that the effects be reduced byother means in the load cell, as the invention may be capable in someinstances of being the only mechanism rejecting the effects.

By "frictional effects", we mean effects that are typically as a resultof imperfect mounting between load cells and load receiving members, aswell as imperfect spring characteristics within the load cells and theload receiving members. These effects typically result in hysteresis andcreep being exhibited in the outputs of the scales subjected to sucheffects.

Therefore, according to one aspect of the invention, a load cell isprovided, which includes a load cell body; a first force sensorpositioned on the load cell body to sense a first force applied to theload cell body along a first force vector, and provide a first outputrepresentative thereof; a second force sensor positioned on the loadcell body to sense a second force applied to the load cell body along asecond force vector, and provide a second output representative thereof;and rejection means, coupled to receive the first and second outputs,for rejecting residual rejected effects due to the second force from thefirst output and generating a third output representative thereof.

According to another aspect to the invention, a method is provided forsensing force with a load cell of the type having a load cell body andfirst and second force sensors positioned thereon to sense forcesapplied to the load cell body respectively along first and second forcevectors. The method includes the steps of sensing a first force appliedto the load cell body along the first force vector with the first forcesensor and generating a first output representative thereof; sensing asecond force applied to the load cell body along the second force vectorwith the second force sensor and generating a second outputrepresentative thereof; and scaling the first output in response to thesecond output to reject residual rejected effects from the first output.

In accordance with an additional aspect of the invention, there isprovided a scale which includes a base; a force receiving member adaptedto receive an object to be weighed; a plurality of load cells coupledbetween the force receiving member and the base to deflect in responseto a force applied to the force receiving membrane by the object, eachload cell having a first output primarily responsive to at least aportion of the force applied by the object, and a second outputprimarily responsive to residual rejected effects sensed by the loadcell; and load cell driving means for providing a force signalrepresentative of the force applied to the force receiving member by theobject. The driving means includes rejection means, coupled to the firstand second outputs of each load cell, for rejecting residual rejectedeffects from the first output of each load cell; cornering means foradjusting the relative sensitivities of the first outputs of the loadcells such that the sum of the first outputs is substantiallyinsensitive to the position of the object on the force receiving member;and summing means for summing the first outputs of the load cells toprovide the force signal.

In accordance with a further aspect of the invention, a method isprovided for weighing an object on a scale of the type including aplurality of load cells coupled between a force receiving member and abase to deflect in response to a force applied to the force receivingmember by the object, each load cell having a first output primarilyresponsive to at least a portion of the force applied by the object, anda second output primarily responsive to residual rejected effects sensedby the load cell. The method includes the steps of rejecting residualrejected effects from the first output of each load cell by scaling thefirst output of each load cell in response to the second output of theload cell; adjusting the relative sensitivities of the first outputs ofthe load cells such that the sum of the first outputs is substantiallyinsensitive to the position of the object on the force receiving member;and summing the first outputs of the load cells to provide the forcesignal.

According to another aspect of the invention, a scale is provided, whichincludes a base; a force receiving member adapted to receive a forceapplied along a force vector; first and second load cells, coupledbetween the force receiving member and the base in parallel and spacedapart on the force receiving member generally in the direction of theforce vector such that the load cells share the force, each load cellproviding an output representative of at least a portion of the forceapplied along the force vector; and load cell driving means forproviding a force signal representative of the force applied to theforce receiving member by summing the outputs of the load cells, thedriving means including frictional effect rejecting means for adjustingthe relative sensitivities of the outputs of the load cells to rejectfrictional effects in the scale.

According to an additional aspect of the invention, a method is providedfor weighing an object on a scale of the type including first and secondload cells coupled between a force receiving member and a base inparallel and spaced apart on the force receiving member generally in thedirection of an applied force vector such that the load cells share anapplied force. The method includes the steps of, on each load cell,providing an output representative of at least a portion of the forceapplied along the applied force vector; adjusting the relativesensitivities of the outputs of the load cells to reject frictionaleffects in the scale; and providing a force signal representative of theforce applied to the force receiving member by summing the outputs ofthe load cells.

These and other advantages and features, which characterize theinvention, are set forth in the claims annexed hereto and forming afurther part hereof. However, for a better understanding of theinvention, and the advantages and objectives obtained by its use,reference should be made to the Drawing, and to the accompanyingdescriptive matter, in which there is described preferred embodiments ofthe invention.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a side elevational view of a lift truck showing a preferredlift truck scale consistent with the principles of the invention.

FIG. 2 is a functional perspective view of a load cell illustratingtypical effects exhibited thereupon, and showing the placement ofsensors on the load cell consistent with the invention.

FIG. 3 is a functional perspective view of a scale mountingconfiguration for illustrating typical effects exhibited thereupon.

FIG. 4a is a functional block diagram illustrating the operation of apreferred dual sensor load cell consistent with the invention.

FIG. 4b is a functional block diagram illustrating cornering correctionin a preferred scale consistent with the invention.

FIG. 4c is a functional block diagram illustrating frictional effectrejection in a preferred scale consistent with the invention.

FIG. 5 is a partially exploded fragmentary perspective view of the lifttruck scale of FIG. 1.

FIG. 6 is a perspective view of one load cell from the lift truck scaleof FIG. 1, with a portion of the shroud and potting cut away.

FIG. 6a is an enlarged fragmentary bottom plan view of the load cell ofFIG. 6, with the shroud and potting removed.

FIG. 6b is an enlarged fragmentary left side elevational view of theload cell of FIG. 6, with the shroud and potting removed and with thecircuit board partially cut away.

FIG. 6c is an enlarged fragmentary top plan view of the load cell ofFIG. 6, with the shroud and potting removed.

FIG. 6d is an enlarged fragmentary right side elevational view of theload cell of FIG. 6, with the shroud and potting removed.

FIG. 7 is a block diagram illustrating the functional components of thelift truck scale of FIG. 1.

FIG. 8 is a schematic of an electrical circuit for the load cell of FIG.6.

FIG. 9 is a schematic of an electrical circuit for the load cell driverof FIG. 7.

FIG. 10 is a partially exploded fragmentary perspective view of analternate lift truck scale mounting configuration consistent with theinvention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Turning to the Drawing, wherein like numbers denote like partsthroughout the several views, FIG. 1 illustrates a lift truck 1 having alift truck scale 10 consistent with the principles of the invention.Lift truck 1 can be any commercially available lift truck, such as theGCX25 manufactured by Clark. Generally, such a lift truck includes achassis 11 supported on a ground surface 13 by wheels 12. An operatorcontrols the lift truck while sitting in seat 14, and loads are carriedby a pair of forks 28 which are supported by crossbars 24 and 26. Forks28 each include a portion 28a which extends generally perpendicular tothe outer surfaces of the crossbars to support one or more objectsthereon. Crossbars 24 and 26 are raised and lowered on a pair ofvertical supports 22, thereby permitting objects such as box 5 to belifted and transported by lift truck 1.

Lift truck scale 10 is generally oriented between crossbars 24 and 26and forks 28 to provide a weight measurement of objects lifted by thetruck. Operator interface and display of the weight measurement ishandled through a controller 30 which is oriented on the lift truck forconvenient access by the truck operator.

The preferred scale and load cells for use therewith are particularlywell suited for use as a lift truck scale for fork lifts and the like.However, it will be appreciated by one skilled in the art that theprinciples of the invention may apply to other weighing andforce-sensing applications. Accordingly, the discussion herein in thecontext of a lift truck scale should not be construed as limiting theinvention solely to such applications.

The preferred scale 10 includes a new dual sensor load cell whichrejects residual rejected effects from the output signal of the loadcell. In addition, the preferred scale uses a plurality of load cellswhich are connected in such a manner that frictional effects arerejected from the output of the scale, and such that the output of thescale is substantially insensitive to the position of objects on thescale. A brief discussion of the principles of operation for each ofthese aspects will be provided prior to discussing the preferredembodiments of the invention.

PRINCIPLES OF OPERATION Conventional Scale Function

When more than one load cell was used in a conventional scale, the loadcells supported the load in parallel. This meant that the total of allof the vertical (load) forces on the scale would have to be equal to theweight applied to the scale for the scale to be accurate. ##EQU1## WhereW was the applied weight, and F_(i) was the vertical force applied tothe i^(th) load cell, which was one of N load cells supporting the loadon the scale. For example, in an accurate scale with four load cells,the total weight applied to the scale would be represented by theequation:

    W=F.sub.1 +F.sub.2 +F.sub.3 +F.sub.4

These forces can be represented by an output from each load cell S_(i),which contribute a signal which was exactly proportional to the verticalforce applied to the load cell. Where a_(i) is the proportionalityfactor between the signal and the vertical force.

    S.sub.i =a.sub.i ·F.sub.i

For the scale to be accurate when the load was placed anywhere on thescale and for any load transfer between the load cells, theseproportionality factors for each load cell would have to be determinedaccurately. This was usually done by applying a known weight at a numberof locations on the scale preferably at locations where most of theweight was borne by only one of the load cells at a time. Theseproportionality factors were implemented by storing numbers representingthem in a digital load cell scale where they were used in the weightcalculation or by adjustments in some elements within the analogconnections of an analog load cell scale where they effected the analogsignals. The assumption was made that the only output from these loadcells was from the vertical force alone. This was correct if onlyvertical forces were allowed to be applied to the load cells or if theload cells were perfect in their ability to sense only the verticalforce component of the forces applied to them. This was not always thecase and some output signal may have been due to other than a verticalforce being applied to the load cell. For example, FIG. 2 illustrates aload cell 400 having a fixed end 402 and a cantilevered end 404 to whicha vertical load is applied. Other load effects, e.g. side moments, endloads, and end moments are also shown. As shown in FIG. 3, a lift truckscale 410 includes a vertical member 418 joined to a fixed member 416through load cells 412 and 414. The non-load effects experienced bythese load cells will vary, e.g. based upon the position of a load suchas at points B and C on forks 419 (which are horizontally offset fromthe connection point A to the load cells).

The non-load effects may be due to deflection of the load cell or itsattachments, or the changes in location of the applied weight, or both,or some horizontal force exerted on the scale from an attachment to thescale. In any case this load cell would then have an output which wasdue to the influence of the sum S_(i), of the forces applied to it.##EQU2## Where m indicates the number of different forces on a load cellincluding the vertical load force and any non-load forces. The verticalforce on load cell number one may be represented by F₁,1, the horizontalforce in the axial direction by F₁,2, the horizontal force perpendicularto the load cell axis by F₁,3, the force effect due to a bending momentabout the vertical to the load cell by F₁,4, etc. Any effect due to theapplied load which causes the load cell to have output may be includedin this summation. Each of the load cells in the scale will have its ownsummation of force effects. ##EQU3## Where:

    S.sub.1 =a.sub.1 ·F.sub.1,1 +a.sub.1 ·F.sub.1,2 +a.sub.1 ·F.sub.1,3 +a.sub.1 ·F.sub.1,4 +a.sub.1 ·F.sub.1,5 + . . . a.sub.1 ·F.sub.1,m

and

    W=F.sub.1,1 +F.sub.2,1 +F.sub.3,1 +F.sub.4,1.

The proportionality was adjusted on this summed output and not just onthe vertical force effect output, since there was no way to determinethe individual force effects. When the load shifts from one load cell toanother due to slippage in the attachment joints or some other type ofmovement, the vertical force may have changed on the load cell but someof the other forces may not, or at least not in the same proportion. Thescale would only be accurate if the other load cells which also hadvertical load shift to or away from them, also had the same verticalforce proportions as this load cell. Otherwise the total output wouldnot be equal to the applied load, and an incorrect scale reading wouldbe experienced. This change in vertical force also would cause a changein other load cell force effects, which would have different proportionsthan the vertical forces. These forces would then contribute toinaccurate representation of the applied load. When the proportions ofthe vertical forces were adjusted the others were also. This presents aproblem since for good scale performance the total of the verticalforces sensed must be equal to the applied load while the total of theother forces sensed must be zero.

Undesired Force Effect Cancellation

For only one non-force effect of significance (besides the desiredvertical force effect), the equations for the vertical force effectsare: ##EQU4## and for the total weight is ##EQU5##

For this weight reading to be independent of the non-load force effects:##EQU6##

This non-load force effect sensitivity may not be the same in all of thetransducers. So, if these forces are due to internal deformation ormovement within the scale assembly, this condition will not be metunless the effects are proportioned so that the summation is made to bezero. Since the vertical force effect proportions must be maintained forinsensitivity to the load position and for load transfer between loadcells, additional signals are required representing this non-load forceeffect which may then be independently proportioned and added to theoutput, allowing adjustments to cancel these residual undesired non-loadeffects on the signal.

To provide this cancellation signal, a sensor circuit substantiallyindependent of the vertical load effect must be used, which has anoutput predominately due to and adjustable for each undesired non-loadeffect to be rejected. This independent signal is then proportioned andadded to the vertical load sensing output, as required, to eliminate thenon-vertical force effects on the total output.

The corrected weight representation would in general be described by theequation: ##EQU7## Where the additional signals are s_(k) and theirproportioning factors are b_(k). These proportioning factors could ofcourse be stored as numbers in a digital system or in the adjustments ofelements in an analog circuit in an analog system. The proportioningfactors are determined by applying the load or force effects severaldifferent ways so as to maximize the output of each of the trim sensorsmore than the others each time. The adjustments of the proportioningfactors are made so that the effects of the residual undesired loadeffect is minimized or cancelled. This may be done using mathematicallinear solutions and test data and determining the cancellation factorsor by adjusting analog elements.

The individual load cells which are to be used in the scale may also beadjusted to have minimal sensitivity to particular undesired loadeffects by having the undesired effects sensed in isolated auxiliarysensing elements on each load cell. The output of these isolated sensorscan then be proportioned and added to the load cells output so as tominimize the effect of these undesired load effects.

For example, FIG. 4a illustrates this dual sensing capability for a loadcell 420 having a shear sensor 422 for sensing the vertical load, and anend effect sensor 424 for sensing the non-load effect. The shear sensor422 may correspond, for example, to a wheatstone bridge connection oftension sensors (e.g. strain gauges) T₁ and T₂ and compression sensorsC₁ and C₂ are shown in load cell 400 of FIG. 2. The end effect sensor424 may correspond to a wheatstone bridge connection of tension sensorsT₃ and T₄ and compression sensors C₃ and C₄. By arranging the tensionand compression sensors in different electrical connections, the endeffect sensor may sense end loads or end moments as desired.

When strain gauges are used for the sensors, an excitation signal mustbe provided thereto. Accordingly, as shown in FIG. 4a, end effect sensor424 may be used as a feedback mechanism, proportioned in block 426, toscale the output of the shear sensor 422.

Thus, the added auxiliary sensor outputs are proportioned so that theycancel the residual undesired force effects. ##EQU8##

When the load cells require adjustment while mounted in a multi-loadcell scale, an adjustment-connection box may be used to minimize orcancel the undesired force effects, e.g. as shown in FIG. 4b. Here, ascale 440 includes four load cells 442, 444, 446 and 448, coupledthrough summers 452, 454 and 458, and scaled by multipliers 450, 451 and456. By applying a load at different points on the scale, both theindividual load cell sensor outputs, as well as the scale multipliers,may be adjusted to correct for undesired non-load effects and to makethe scale insensitive to load position.

Frictional Effect Cancellation

Utility may also be gained when more than one load cell supports aportion of a total load at a single point on a plane orthogonal to theload vector, e.g., in the orientation of load cells 412 and 414 in FIG.3.

The sum of the outputs from the load cells supporting the load at onepoint will represent the output for that point. ##EQU9## Where irepresents the number of one point supports connected to the platform, krepresents one of K load cells supporting that point, S_(i) representsthe composite output of the load cells supporting the load force F_(i)at point supports connected to the platform, S_(i),k represents theindividual load cell output signals and a_(k) represents the proportioneach load cell supports of the force F_(i).

Since the sum of the individual forces on the load cells in a multi loadcell scale must be equal to the total load force applied to the scale,any movement of the load within the load cells will not effect thattotal. If, however, the load cells do not have equal sensitivity to theload, the total output signal from the load cells may change due toredistribution of the load force applied. If the load cells are alsosensitive to non-load forces, some of the output signal from the loadcells may be due to these non-load forces as well. ##EQU10##

The total of all of the load cell outputs may not always be equal to thesame proportional relationship to the total applied load unless non-loadforce effects on the outputs and the effects of redistribution cancel.##EQU11##

The proportionality factors a_(i) are constant and cannot change for ascale to be accurate. This means that the sensitivity of these constantsto any change in any of the variables: a_(i),k (proportional loadshare), F_(i),l (load force) or F_(i),j (non-load force), x and y (loadposition), X (fork position), and t (time) must be zero.

More specifically, with load cells 412 and 414 of FIG. 3, there are onlytwo composite outputs, S₁ and S₂, where each is the result of the outputfrom only two load cells.

    S.sub.1 =a.sub.1 ·F.sub.1,1 =(a.sub.1,1 +a.sub.1,2)·F.sub.1,1 +(b.sub.1,1,2 -b.sub.1,2,2)·F.sub.1,2

and

    S.sub.2 =a.sub.2 ·F.sub.2,1 =(a.sub.2,1 +a.sub.2,2)·F.sub.2,1 +(b.sub.2,1,2 -b.sub.2,2,2)·F.sub.2,2

Where the weight on the scale W is represented by the sum of thecomposite outputs.

    W=F.sub.1,1 +F.sub.2,1

The load forces F₁,1 and F₂,1 may be related to x, the horizontal loadposition parallel to the load cell support plane, where R is thehorizontal distance between the composite load cell supports. ##EQU12##

The non-load forces F₁,2 and F₂,2 may be related to y, the horizontalload position from the load cell support plane and, H, the verticaldistance between the load cells in each composite load cell and the forkpositions X₁ and X₂, horizontal positions parallel to the load cellsupport plane.

    F.sub.1,2 =F.sub.1,1 ·(y/H+c.sub.1,1 ·X.sub.1 +c.sub.1,2 ·X.sub.2)

and

    F.sub.2,2 =F.sub.2,1 ·(y/H+c.sub.2,1 ·X.sub.1 +c.sub.2,2 ·X.sub.2)

Without additional sensor inputs as in the dual force sensing load cellthe sensitivity of the x, y load position may be eliminated by properadjustment of the a_(i),j load share factors by span adjustment of theindividual load cell outputs. Otherwise, with some sensitivity to loadposition allowed, the sensitivity to fork position may be eliminated byproper adjustment of the a_(i),j load share factors by span adjustmentof the individual load cell outputs.

The load share factors a_(i),j may change as the load shifts over time.This would present itself in the individual load cell as an elasticcreep. Under these conditions the individual load cell outputs signalswould not be stable. This presents time dependent factors.

    S.sub.1 =[a.sub.1,1 ·(α.sub.1,1 +β.sub.1,1 ·t+γ.sub.1,1 ·t.sup.2 + . . . )+a.sub.1,2 ·(α.sub.1,2 +β.sub.1,2 ·t+γ.sub.1,2 ·t.sup.2 + . . . )]·F.sub.1,1

and

    S.sub.2 =[a.sub.2,1 ·(α.sub.2,1 +β.sub.2,1 ·t+γ.sub.2,1 ·t.sup.2 + . . . )+a.sub.2,2 ·(α.sub.2,2 +β.sub.2,2 ·t+γ.sub.2,2 ·t.sup.2 + . . . )]·F.sub.2,1

These time dependent factors may be cancelled from the composite loadcell output by properly adjusting the load share factors so that thedependency factors cancel.

    a.sub.1,1 ·(β.sub.1,1 ·t+γ.sub.1,1 ·t.sup.2 + . . . )+a.sub.1,2 ·(β.sub.1,2 ·t+γ.sub.1,2 ·t.sup.2 + . . . )+0

and

    a.sub.2,1 ·(β.sub.2,1 ·t+γ.sub.2,1 ·t.sup.2 + . . . )+a.sub.2,2 ·(β.sub.2,2 ·t+γ.sub.2,2 ·t.sup.2 + . . . )=0

When load share factors a_(a),j change with load cycling, the staticfriction prevents the increasing force from matching the decreasingforces. This would present itself in the individual load cell as statichysteresis. Under these conditions the individual load cell outputsignals would not repeat their values when the load cycle is reversed.This presents load change dependent factors.

    S.sub.1 =[a.sub.1,1 ·(α.sub.1,1 +μ.sub.1,1 ·ΔF.sub.1,1 +σ.sub.1,1 ·ΔF.sub.1,1.sup.2 + . . . )+a.sub.1,2 ·(α.sub.1,2 +μ.sub.1,2 ·ΔF.sub.1,1 +σ.sub.1,2 ·ΔF.sub.1,1.sup.2 + . . . )]·F.sub.1,1

and

    S.sub.2 =[a.sub.2,1 ·(α.sub.2,1 +μ.sub.2,1 ·ΔF.sub.2,1 +σ.sub.2,1 ·ΔF.sub.2,1.sup.2 + . . . )+a.sub.2,2 ·(α.sub.2,2 +μ.sub.2,2 ·ΔF.sub.2,1 +σ.sub.2,2 ·ΔF.sub.2,1.sup.2 + . . . )]·F.sub.2,1

These load change dependent factors may be cancelled from the compositeload cell output by properly adjusting the load share factors so thatthe dependency factors cancel.

    a.sub.1,1 ·(α.sub.1,1 +μ.sub.1,1 ·ΔF.sub.1,1 +σ.sub.1,1 ·ΔF.sub.1,1.sup.2 + . . . )+a.sub.1,2 ·(α.sub.1,2 +μ.sub.1,2 ·F.sub.1,1 +σ.sub.1,2 ·ΔF.sub.1,1.sup.2 + . . . )=0

and

    a.sub.2,1 ·(α.sub.2,1 ·ΔF.sub.2,1 +σ.sub.2,1 ·ΔF.sub.2,1.sup.2 + . . . )+a.sub.2,2 ·(α.sub.2,2 +μ.sub.2,2 ·ΔF.sub.2,1 +σ.sub.2,2 ·ΔF.sub.2,1.sup.2 + . . . )=0

The operation of this type of scale is illustrated in FIG. 4c, where ascale 430 includes top and bottom load cells 432 and 434 summed at 438,with one output scaled via block 436. Of course, in the scale shown inthis FIG., as well as the load cells and scale of FIGS. 4a and 4b, mayproviding scaling by scaling either, or both signals prior to theirsummations.

Tiltable Scale

It has been found that a tiltable or portable scale may also be producedwhich weighs accurately when tilted consistent with the invention. Thisscale includes at least one tilt sensor and electronic circuits with theability to process the signals from a tilt sensor and a load sensor. Theload sensor may contain any number of force sensors as required tosupport the load on the load support from the base. The electroniccircuits process the signals from the force sensors and tilt sensors andprovide the correct signal relative to the weight applied to the scale,independent of the effects of tilting the scale from its level state.

It is known that, if uncompensated, tilting scales cause variations ofthe force perpendicular to the load support plane, and stationary scalesmust be level to weigh accurately. If the angle of tilt is known theforce on the scale's load support may be predicted as a function of thisangle. This prediction may then be used to calculate the weight.##EQU13##

Here the force perpendicular to the scale support plane, sensed by thedirectionally sensitive load sensor, is F₁ the tilt angle is Θ, the loadsensor output signal is S, and the force sensitivity factor for loadsperpendicular to the plane of the load support is A₁.

If the scale may tip about more than one axis of tilt, a tilt sensorwith more than one axis of tilt sensitivity is required or two tiltsensors are required. When the tilt sensors are oriented perpendicularto one another, the effect of the angle, Θ, is a function of the twoangles sensed, φ₁ and φ₂. ##EQU14##

If the load sensor is sensitive to forces which are not perpendicular toits support plane, another variation in the scale signal may bedetected, where A₂ and A₃ are force sensitivity factors of the loadsensor to forces tangent to the load support plane.

    S=A.sub.1 ·W·cos Θ+A.sub.2 ·W·sin φ.sub.1 +A.sub.3 ·W· sin φ.sub.2

This may be the case when a dual force sensor load cell is used in ascale or the load cell used has sensitivity to forces tangent to thescale load support plane. It may be shown that the influence of thesetangent sensitivities may be represented by phase shifts δ₁ and δ₂ inthe angle sensor inputs and altered sensitivity factor A₄. ##EQU15##

If the load sensor provides separate force sensitivity outputs in twodirections not in line with the primary force sensitivity direction,these outputs may be used to determine the angles of tilt and be used asangle sensors. When the two force sensitivity directions are orthogonalto the primary and each other they define a coordinate system and may beused directly to measure the applied force independent of the angle totilt. ##EQU16## where the forces are measured by calibrating theindividual force outputs. The angles of tilt may be determined:##EQU17##

MECHANICAL CONFIGURATION

The mechanical components of the preferred lift truck scale 10 areillustrated in FIG. 5. It is preferable to design scale 10 such that itis easy to install and remove from a standard lift truck withoutsubstantial modification to the truck. In the preferred embodiment, thescale simply hangs off of the existing crossbars 24, 26 of the truck inthe same manner as standard forks. No other modification to the lifttruck, other than installation of the housing for controller 30, routingof wires between the scale and the controller, and connecting thecontroller to the battery of the lift truck, is typically required.

As shown in FIG. 5, the crossbars 24, 26 on lift truck 1 are laterallyextending and vertically spaced from one another. Crossbar 24 has aplurality of notches on it stop surface for receiving forks 28 atoperator-selected widths. Lift truck scale 10 includes first and secondvertically oriented mounting plates 40 and 60 which are interposedbetween the crossbars and forks of a standard lift truck, and arecoupled through a plurality of load cells 100, 102, 104 and 106.

First plate 40 operates as the base of the scale and includes a pair ofhooks 42 which engage crossbar 24 such that plate 40 is supported by thecrossbar. A pair of removable hooks 44 bolt onto the underside of plate40 with bolts 45 to fix the plate on crossbar 26 in a secure manner.Installation and removal of scale 10 to or from the lift truck isrelatively simple, since only the attachment or detachment of removablehooks 44 to or from plate 40 is generally required.

Second plate 60 is mounted to first plate 40 through load cells 100,102, 104 and 106. Second plate 60 includes notches on its top surfacesimilar to t hose on crossbar 24, thereby enabling forks 28 to beinstalled thereupon at a plurality of lateral position similar to themanner in which they may be installed on the crossbar of a conventionallift truck. Second plate 60 and forks 28 form the force receiving memberor structure for scale 10 since it is this structure which applies theforce from an object support by the forks across the load cells forweighing. However, it will be appreciated that in differentapplications, the base and force receiving structure may vary consistentwith the application. For example, for truck or platform-type scales,the force receiving structure will typically be a horizontal platform,while the base will be a housing or other structure upon which the loadcells are mounted.

A junction box 20 is mounted to plate 40 facing plate 60 and isprotected behind a cover attached to plate 60. Junction box 20 housesthe load cell driver 200 and the tilt sensors 304, 306, which aredescribed below in conjunction with FIG. 7. Junction box 20 ispreferably a weather resistant housing to protect the electricalcircuitry housed therein from the environment.

Load cells 100, 102, 104 and 106 are oriented generally perpendicular tothe plates and are disposed in two laterally spaced vertical columns.When viewed from the front of the scale, load cell 100 is a top leftload cell, load cell 102 is a top right load cell, load cell 104 is abottom left load cell, and load cell 106 is a bottom right load cell.

Load cell 100, which is also exemplary of load cells 102, 104 and 106,is shown in FIGS. 6 and 6a-d. Load cell 100 generally includes athreaded shank 110, a sensor section 112 and a flange 114. Shank 110defines a neutral or longitudinal axis 130 for the load cell, and itincludes a ring 118 which acts as a stop when the load cell is installedon plate 40. A nut 126 and washer 128 are used to secure the load cellto plate 40, as will be discussed below. In addition, load cell 100includes a flange 114 which is generally orthogonal to the longitudinalaxis 130 of shank 110 and is adapted to mount to plate 60 using aplurality of bolts 70 secured through apertures 68 into threaded holes115 (shown in FIG. 5).

Sensor section 112 is disposed between stop 118 and flange 114. Thissection includes a deformable member 116 which is designed to deform ordeflect in response to a force applied along a first, desired forcevector, which in this case is a vertically oriented force due to gravitydesignated by arrow 113. Deformable member 116 is also designed toreject at least some of the forces applied on other force vectors,thereby substantially isolating the desired forces to be sensed. Thedeformable member is milled or otherwise formed with a narrowed portiondefining a pair of opposing planar side surfaces 132, 134, whereby avertical force applied in a direction parallel to the surfaces (i.e.,along arrow 113) will introduce a measurable shear across the sensorsection of the load cell. Top and bottom surfaces 137, 138 are orientedgenerally orthogonal to the side surfaces 132, 134.

The body of load cell 100 is preferably formed of tool steel. Othersuitable materials used for load cells and the like including othermetals, metal alloys, composite materials, etc. may also be used.

A first force sensor 140 is mounted in sensor section 112 to measure theshear forces applied to deformable member 116 by a vertical load. Forcesensor 140 in the preferred embodiment includes four force transducers,preferably strain gauges T1, T2, C1 and C2, which are connected in awheatstone bridge configuration. Two of the strain gauges T1, T2 areplaced in tension by an applied force, and the other gauges C1, C2 areplaced in compression.

The strain gauges are of the type which operate by varying in resistancein response to a strain applied to the gauge. For example, one suitablestrain gauge is the constantan polyimide strain gauge manufactured byMicro Measurements. The strain gauges typically provide a resistancewhich varies in the range of 120 to 5000Ω. Other commercially availablestrain gauges may also be used in the alternative.

The strain gauges in sensor 140 are preferably matched to minimize anyspan differences therebetween. In addition, the strain gauges areoriented on the deformable member 116 to be responsive primarily to theshear forces applied along the first force vector 113. Tension gauges T1and T2 are oriented on the opposing planar surfaces 132, 134, and aretilted 45° from the neutral axis 130 such that they are placed intension in response to shear forces. Compression gauges C1 and C2 arealso disposed on planar surfaces 132 and 134, but are oriented 90° fromthe tension gauges such that they are placed in compression in responseto the shear forces.

In this configuration, when the strain gauges are connected in awheatstone bridge configuration (discussed in greater detail below), theoutput of the bridge circuit will be primarily responsive to force alongthe desired force vector 113, with other forces being significantlyrejected from the output. However, due to errors as a result ofmachining tolerances, strain gauge mismatches, mounting imperfections,etc., other forces, designated "residual rejected effects" will stilltypically affect the output of first sensor 140. Accordingly, a secondforce sensor 150 is also included on load cell 100 to isolate andmeasure one or more of these residual rejected effects.

In the preferred embodiment, second force sensor 150 includes fouradditional strain gauges T3, T4, C3 and C4 also connected in awheatstone bridge circuit. The gauges in sensor 150 are preferablymatched with one another, and may be also matched with those in sensor140.

Strain gauges T3 and T4 are oriented to be placed in tension and straingauges C3 and C4 are oriented to be placed in compression as a result ofnon-load force effects on one or more force vectors other than vector113. For example, as shown in FIGS. 6a and 6c, gauges T3 and T4 areoriented on top and bottom surfaces 137 and 138 of deformable member116, and are oriented generally along the longitudinal axes thereof. Thecompression gauges C3 and C4 are also oriented on the top and bottomsurfaces, but at 90° from the tension gauges.

In the configuration shown for sensors T3, T4, C3 and C4, two differenteffects may be measured if desired. To measure end loads, which areprimarily horizontal forces oriented transverse to the first forcevector, the gauges in sensor 150 may be arranged in a wheatstone bridgeconfiguration as shown in FIG. 8. Alternatively, to measure end moments(see FIG. 2), gauges C4 and T4 may be switched in the wheatstone bridgecircuit to measure these effects instead.

It will also be appreciated that additional sensors may be disposed onthe load cell to measure more than one effect along more than one forcevector. Moreover, the secondary sensors may also be disposed on otherstructures in the scale, for example on separate flexures connecting thetwo plates. However, in the preferred embodiment the secondary sensorsare disposed on the same member as the primary force sensors.

Several other components are mounted to load cell 100. For example, acircuit board 136 is mounted generally parallel and offset to the planarsurfaces 132. The circuitry found on this board is discussed below inrelation to FIG. 8. In addition, a lead wire 124 is mounted to thecircuit board and carries signals to and from the load cell. Moreover, apair of thermistors VR1 and VR2 are mounted to planar surface 132. Thesethermistors are resistive elements which change in resistance inresponse to temperature variations. The use of these thermistors willalso be discussed below in relation to FIG. 8.

The sensors and other components of load cell 100 are preferably housedwithin a shroud 120 to protect these elements from the environment.Preferably, a potting material 122, such as polyurethane, is alsodisposed within shroud 120 to assist in the protection of the elementsof load cell 100 from the environment, and to minimize any temperaturevariations experienced by the scale.

Returning to FIG. 5, load cells 100, 102, 104 and 106 are mountedbetween plates 40 and 60 of scale 10. Each load cell is preferablymounted first to second plate 60 by inserting threaded shank 110 throughaperture 64 in second plate 60 such that the flange 114 is received incountersunk recess 66. A plurality of bolts 70 then fit into thecorresponding apertures 68 and threaded apertures 115 respectively insecond plate 60 and flange 114 to secure the load cells to the secondplate.

Once the load cells are mounted to plate 60, they may be mounted tofirst plate 40 by installing threaded shanks 110 through apertures 46 infirst plate 40, then securing the threaded shanks by nut 126 and washer128. Countersunk recesses 48 are provided proximate the apertures 46 onthe side facing the crossbars such that nut 126 is flush or recessedwith respect to the surface of plate 40.

With the configuration shown herein, plate 40 is engaged on ring 118 ofeach load cell, and flange 114 is engaged in recess 66 in plate 60.Consequently, the connection between plates 40 and 60 provides arelatively thin profile with minimum separation between the plates. Thisis important because the greater the thickness of the scale, the morethe capacity of the lift truck is decreased due to the greaterseparation between the forks and the "fulcrum" of the lift truck,typically the front wheels.

Other mounting configurations may be used in the alternative. Forexample, as an alternative to nuts 126, jamnut tensioners, such as theSJ models manufactured by Super Bolt Inc., may be used. These tensionersinclude an annular array of tensioning set screws which are tightenedafter the tensioner is threaded onto shank 110. Tensioners of this typemay provide less slippage in the mounting and therefore more perfectloading of the shank.

In addition, the single plate mechanical configuration shown in theaforementioned Bradley patent may also be used as an alternative to thedual plate mechanism disclosed herein. Further, more or less numbers ofload cells may be used between the plates, and one or more flexures mayalso be disposed between the plates to bear some of the load.

As an example, FIG. 10 shows an alternate mechanical coupling which usesonly two load cells along with two or four flexures. As shown in thisFig., first and second plates 40' and 60' each include six apertures46', 64' in two laterally spaced vertical columns. A pair of load cells100' and 102' are mounted in the center apertures in each verticalcolumn. In addition, four flexures 103 are mounted in the remainingapertures to bear some of the load between the plates. Each flexure hasa similar structure to the load cells, particularly in the mountinghardware for the purposes of interchangeability, but does not includeany of the sensors or other electrical components. The flexures may benarrowed (i.e., to bring the opposing planar surfaces closer together)such that each flexure bears less of the overall force, therebyattenuating less of the overall force borne by the scale. In addition,the flexures are preferably rotated about 90° from the load cells alongtheir longitudinal axes so that the opposing planar surfaces thereon arebasically horizontal to resist any lateral forces.

This type of alternate configuration has the advantage of providing areduced number of parts, greater part interchangeability, and a reducedcost because of the use of only two load cells. In addition, the use ofthe flexures may increase the overall capacity of the scale. Further, itis foreseen that the same plates 40' and 60' could be used for two,four, or even six load cell scales without substantial modification,since any unused apertures may be filled with flexures if desired.Consequently, different capacity or accuracy scales could bemanufactured using the same basic mechanical components.

ELECTRICAL CONFIGURATION

The primary electrical components of the preferred lift truck scale 10are shown functionally in FIG. 7. The scale electronic components arehoused in three primary locations. In particular, the circuitry for eachload cell 100, 102, 104 and 106 is primarily disposed within the sensorsection of each load cell, as described above. The load cell driver 200is preferably located within junction box 20, along with a pair of tiltsensors 304 and 306 which are used in tilt adjustment. Finally, thehigher level processing functions of the scale are performed bycontroller 30 which is found in a separate housing that is accessible toan operator (e.g., in the position shown in FIG. 1). Controller 30generally provides the load cell excitation function 300, the tiltadjustment function 302, the weight processing function 308, the userinput function 312 and the display function 310.

Load Cell Circuitry

FIG. 8 shows the preferred circuitry for load cell 100. The circuitryfor the other load cells is substantially the same. Electricalcommunication with the load cell is performed through a lead wire 124which includes a shield line, three input lines (+LOAD EXC, +TRIM EXC,and -EXC), and two output lines (+OUT and -OUT). The shield line issimply tied to the body of the load cell to shield unwanted interferencefrom the other signal lines.

The first sensor 140 is designated the "load sensor" as this sensor isprimarily responsive to the load applied to the load cell. Sensor 140 ispreferably a resistive network formed by the wheatstone bridge couplingof strain gauges T1, T2, C1 and C2. In addition, a pair of thermistors,TCR1 and TCR2 are optionally included in series with gauges T2 and C2 toprovide zero compensation for temperature effects on the output of thebridge. Thermistors TCR1 and TCR2 are of nominal resistance (about 1Ω),such as no. 34 copper wire manufactured by Micro Measurements.

Second sensor 150 is designated the "trim sensor" as this sensor sensesone or more residual rejected effects and "trims" the response of thefirst sensor to reject these effects from the output of the load cell.Sensor 150 is formed by the wheatstone bridge coupling of strain gaugesT3, T4, C3 and C4. As discussed above, this configuration sensesprimarily end loads oriented transverse to the primary force vector.Optionally, to measure end moments, strain gauges C4 and T4 may beswitched in the bridge.

Sensors 140 and 150 each have two input lines (+EXC and -EXC) forming aninput for receiving an excitation voltage, and each have two outputlines (+OUT and -OUT) forming an output for providing an output signalrepresentative of the force applied to the sensor. Sensors 140 and 150are generally coupled in parallel, whereby the output of sensor 150 willin effect scale the output of sensor 140 to compensate for residualrejected effects.

The +EXC inputs of sensors 140 and 150 are coupled to two separateexcitation signals from lead wire 124. Sensor 140 is coupled to the+LOAD EXC excitation signal through resistor R5 in series with theparallel network of resistor R1 and variable resistor VR1. The -EXCinput of sensor 140 is coupled to the -EXC excitation signal in leadwire 124 through resistor R6 in series with the parallel network ofresistor R2 and variable resistor VR2.

Resistors R5 and R6 provide span adjustment for the output of the loadcell, and they are preferably fixed resistances in the range of 0 to100Ω. Fixed resistors R1 and R2 and variable resistors VR1 and VR2provide temperature compensation means for the load cell such thattemperature variations will not affect the span or sensitivity of theload cell. Variable resistors VR1 and VR2 are preferably thermistors,such as bondable nickel resistance elements manufactured by MicroMeasurements, with a maximum resistance in the range of 0 to 140Ω. Fixedresistors R1 and R2 are typically in the range of about 200Ω.

The +EXC and -EXC inputs of sensor 150 are generally coupled in parallelto sensor 140. The +EXC input of sensor 150 is coupled to a excitationsignal +TRIM EXC from lead wire 124, which is separately driven from theload sensor excitation +LOAD EXC in the preferred embodiment tofacilitate the rejection of residual rejected effects from the load celloutput, as will be discussed below. The -EXC input of sensor 150 isdirectly coupled to the -EXC input of lead wire 124.

The output signals of sensors 140 and 150 are also coupled in parallelsuch that the signal present on the outputs of sensor 150 functions toscale the output of sensor 140 in response to residual rejected effects.The +OUT and -OUT inputs of sensor 140 define a first output for theload cell and are directly coupled to the +OUT and -OUT signals of leadline 124. The +OUT and -OUT outputs of sensor 150 define a second outputfor the load cell and are coupled to the +OUT and -OUT signals of leadline 124 through resistors R3 and R4, respectively. Resistors R3 and R4are fixed resistors which generally attenuate the output of sensor 150compared to sensor 140, and will typically have resistances in the rangeof 5-500 kΩ. The overall output of the load cell, a third output, isformed across lines +OUT and -OUT of lead line 124. This resulting thirdoutput across +OUT and -OUT of lead line 124 is responsive to the forcesensed by sensor 140, and is compensated for residual rejected effectsby the output of sensor 150.

In the preferred embodiment, the residual rejected effects rejectionmeans includes a scaling means for scaling the relative outputs of thefirst and second sensors 140 and 150. This scaling is preferablyperformed (1) through selection of resistors R3 and R4, and (2) throughcontrolling the relative magnitudes of the excitation signals +LOAD EXCand +TRIM EXC. Resistors R3 and R4 are preferably fixed, therebyproviding a "coarse" adjustment of the outputs. A "fine tuning" of thisadjustment is performed by controlling the excitation voltages to eachbridge.

As an alternative, resistors R3 and R4 may be variable resistors, withsensors 140 and 150 running off of the same excitation signal. Inaddition, the scaling may be performed at the excitation voltage, at theoutputs, or both (as in the case of the preferred embodiment) toeffectuate the rejection of residual rejected effects. The scaling maybe performed wholly within the load cell circuitry, or may useadditional circuitry (such as circuit 200 as will be discussed below).Further, either sensor 140 or 150, or both, may be scaled to effectuatethe relative scaling of the outputs of the sensors. Furthermore, thescaling may be performed by attenuation, (as with the resistorsdisclosed herein), or through amplification (e.g. using an operationalamplifier). In general, any mechanism for scaling the relativesensitivities of the two sensors may be used consistent with theinvention.

It will be appreciated that different sensors and circuitry may be usedin the preferred load cell consistent with the invention. For example,the strain gauges may be arranged to sense different effects alongdifferent force vectors, including end moments, side moments, twistingmoments, lateral forces, horizontal forces, etc. In addition, othertypes of force transducers may be used as alternatives to strain gauges,including various capacitive transducers, resistive transducers,resonating transducers (e.g., tuning forks, vibrating strings,piezoelectric crystals), optical sensors, solid state strain sensors,etc.

In addition, it will be appreciated that scaling of the relativesensitivities of the first and second sensors may be performed throughother analog means than described herein, and may also be performedthrough various digital signal processing algorithms, using varioushardware configurations and/or software routines. Such digitalmanipulation of the signals would be particularly advantageous in thecase where digital or resonating transducers were used, since digitalsignals are inherently less susceptible to errors than analog signals.

Other modifications to the preferred load cell may also be madeconsistent with the invention.

Load Cell Driver Circuitry

FIG. 9 shows the load cell driver circuit 200 in greater detail. Drivercircuit 200 essentially operates as a routing circuit for passingexcitation, output and shield signals to and from controllers 30 andeach of the load cells 100, 102, 104, and 106. Driver circuit 200provides a common excitation signal to the load cells, scales theexcitation signals to correct for various effects, and returns a commonoutput signal representing generally the sum of the forces sensed by theload cells, corrected for various extraneous effects.

As shown in FIG. 9, the common +EXC signal from controller 30 is used todrive both the load and trim bridges on the load cells through lines+LOAD EXC and +TRIM EXC, respectively. For the load bridges, apotentiometer (variable resistor) 203 is used to apportion the currentfrom the +EXC signal provided by the controller to scale the relativemagnitudes of the excitation voltages supplied to the load cells on leftand right halves 201, 202 of circuit 200. Left and right circuit halves201 and 202 correspond respectively to the left and right groups of loadcells. Accordingly, potentiometer 203 is designated the left/rightbalance pot.

A pair of potentiometers 204, 206 are treed off of left/right balancepot 203 in a cascaded arrangement to apportion the current from pot 203to scale the relative magnitudes of the excitation voltages supplied tothe top and bottom load cells on each half of the circuit. Potentiometer204 scales the excitation voltages of load cells 100 and 104, and isdesignated the left top/bottom balance pot. Potentiometer 206 scales theexcitation voltages of load cells 102 and 106, and is designated theright top/bottom balance pot.

As a result, potentiometers 203, 204 and 206 form a cornering means, ormore particularly, an excitation scaling means, for scaling the relativemagnitudes of the +LOAD EXC signals of each individual load cell 100,102, 104 and 106. Alternatively, separate variable resistors could betied to each +LOAD EXC signal to provide the individual sensitivityadjustment of the signals. However, such a configuration would be moredifficult to calibrate than the cascaded group of three potentiometers203, 204, 206 used in the preferred embodiment.

For the trim bridges of the load cells, a pair of potentiometers 208 and210 form an input scaling means and are used to apportion the currentfrom the +EXC signal provided by the controller to scale the relativemagnitudes of the +TRIM EXC signals provided to the top and bottom loadcells on each half 201, 202 of the circuit. Potentiometer 208 scales the+TRIM EXC signals provided to load cells 100 and 104 and is designatedthe left top/bottom trim pot. Potentiometer 210 scales the +TRIM EXCsignals provided to load cells 102 and 106 as is designated the righttop/bottom trim pot.

The remaining signal lines in circuit 200, -EXC, +OUT, -OUT, and SHIELD,are routed directly between controller 30 and each of the load cells100, 102, 104 and 106. The common output signal sent to the controller,across +OUT and -OUT, represents the sum of the scaled individual loadcell outputs. However, a zeroing means, potentiometer 212 (designated asthe zero pot), is interposed between the +EXC and -EXC signals, and isdriven by the -OUT signal from controller 30. Zero adjustment for thescale may be provided by adjusting zero pot 212, which may be beneficialfor achieving the maximum usable range when performing analog to digitalconversion.

Potentiometers 203, 204, 206, 208, 210 and 212 are preferably multiturnpotentiometers having a range of 20 to 500Ω. Other resistances, however,may be used as required by other designs.

The potentiometers as shown permit the scale to have residual rejectedeffect rejection, frictional effect rejection and cornering correctionfor the scale output. However, these potentiometers must be calibratedprior to usage before these advantages are realized by the scale.

To properly calibrate the potentiometers, an operator first must takeincreasing and decreasing loads at a single position on the scale, andadjust the left and right top/bottom balance pots 204 and 206 such thatany hysteresis effects are nulled (i.e., so that an output graph of theincreasing loads matches that of the decreasing loads).

Next, an operator applies one load directly over one of the forks (e.g.,on the left side) relatively close to plates 40 and 60 (an "aft"position), then applies another load relatively far from the plates (a"fore" position). The left top/bottom trim pot 208 may then be adjustedso that the difference in the outputs is zero.

Next, loads are applied at fore and aft positions on the right sidefork, and the right top/bottom trim pot 210 is adjusted to zero thedifference in the outputs in the same manner as above for the left sidepot.

Next, a load is applied on one side (i.e., proximate one fork) andanother load is applied on the other side (i.e. proximate the otherfork). The left/right balance pot 203 is then adjusted until thedifference in outputs is zero.

Once the balance and trim pots are calibrated, the zero pot 212 may beadjusted to provide a zero output when no external force is applied tothe scale. Then, after calibration, the resulting output across the +OUTand -OUT terminals returning to controller 30 will generally be the sumof the outputs of the load cells, corrected for residual rejectedeffects and frictional effects, and cornered to eliminate any positionsensitivity. The resulting output may then be transmitted to controller30 to perform various scaling and adjustment functions to provide auseful display to the operator of the lift truck.

Various modifications to circuit 200 may be made consistent with theinvention. For example, different mechanisms of scaling the excitationsignals may be used. In addition, the outputs could be scaled inaddition to or in lieu of scaling the excitation signals. Moreover,circuit 200 may be implemented digitally using dedicated hardware, ormay be implemented within controller 30 as a separate digital signalprocessing routine. Other modifications will be apparent to one skilledin the art.

Scale Controller

As discussed above, controller 30 is located at a convenient positionfor viewing by the operator of the lift truck. Controller 30 provides anoverall user interface for controlling the operation of the scale. Eachof the primary functions of the preferred controller are illustrated inFIG. 7. However, the actual hardware components and software routinesnecessary to execute these functions are not discussed, as many of thefunctions are typically provided in conventional scale designs and couldbe easily implemented by one skilled in the art without undueexperimentation.

One function provided by controller 30 is load cell excitation in block300. Block 300 provides regulated +EXC and -EXC lines to drive theindividual load cells through driving circuit 200. The excitationsignals generated by block 300 may be alternating or direct current,typically between 3 and 30 volts. The manner of driving strain gaugebridges to sense force is generally known in the art, and thus, theelectrical components necessary to provide the desired driving signalsare not described herein.

Controller 30 accepts the output from load cell driver 200, which isfirst converted to digital form in an analog to digital conversion block301 known in the art. As a result, the analog voltage present across the+OUT and -OUT terminals of circuit 200 is converted to a digital value.

Next, a tilt adjustment block 302 takes the digital output of block 301and corrects the output for any tilt in the scale. This optionalfunction, which is primarily useful in portable scales and the likewherein the orientation of the scale may change in operation, makes suchscales more flexible since they do not need to be leveled prior toweighing. This is an especially beneficial feature on a lift truckscale, since lift trucks may be used on inclines, and additionally,since some lift trucks have the ability to tilt the forks from a neutralposition.

It will be appreciated that as a surface tilts away from pure level, thevertical component of the force applied by an object resting on thesurface will vary. Consequently, a tilt adjustment mechanism may beneeded to compensate for the change in the verticle component of theforce as a function of the degree of tilt of the surface.

As shown in FIG. 7, tilt sensors 304 and 306 are housed within junctionbox 20. Sensor 304 is oriented to measure the fore/aft tilt of thescale, and sensor 306 is oriented to measure the left/right tilt of thescale. Therefore, in combination, the outputs of sensor 304 and 306 maybe able to determine the three dimensional tilt of a surface.Accordingly, sensors 304 and 306 must be positioned on the scale in amanner to sense the degree of tilt in both directions. While junctionbox 20 is a convenient location due to its direct mount on the scale andits protection from the environment, other positions for the sensors mayalso be envisioned.

Sensors 304 and 306 are preferably angular position sensors such asclinometers manufactured by Lucus. Other commercially available sensorsmay also be used.

The adjustment of an output signal for the degree of tilt of a surfaceis performed as described above in the principles of operation section,with the angles sensed by sensors 304 and 306 applied as angles φ₁ andφ₂ in the tilt equation developed above. Other known tilt adjustmentroutines may also be used consistent with the invention.

Calibration of the scale for tilt is necessary to adjust the scaleelectronic circuitry so that the scale output indicator indicates thecorrect weight when the scale is used within its operating range. Thescale may also only display an output when operating within apredetermined tilt range.

Calibration may be performed in any manner known in the art to generatesuitable constants for the aforementioned equation. For example, tocalibrate the scale, known calibration loads may be placed on the scalewhen the scale is tilted in each direction of sensitivity to variousdegrees. The output signal may be recorded for these various loads andtilt angles and the calibration load values entered. Factors for thecorrection of linearity errors, tilt errors, and span may then bedetermined by any numerical analysis technique suitable for determiningequations of functions to fit acquired data. The scale may also beoperated to determine the limits of the tilt angles while maintaining anaccurate scale reading. Scale output blanking angle set points may thenbe programmed or set so that the scale will only display correct weightreadings.

After the digital output signal has been corrected for tilt, additionalweight processing takes place in function block 308 to provide a userreadable weight reading. Block 308 may rely on user input from block312, which may include, for example, a keypad which permits a user toaccess various functions on the scale. Block 308 may also rely on aweight display 310, e.g. an LED or LCD display panel, for displaying themeasured weight and other valuable information to the operator.

Block 308 may perform many functions for providing useful weightinformation to an operator. The functions provided by block 308 mayinclude spanning the force reading to real world units (e.g., pounds orkilograms), averaging multiple samples over time to reduce transienterrors, providing a tare function, a zero reset function, etc. Inaddition, block 308 may include diagnostic or calibration modes toconfigure and characterize the scale. Moreover, various data loggingfunctions may be used to record and tabulate weight reading histories,and output functions may be included to transmit weight readings to anexternal source such as a printer or a host computer. Other scalefunctions, known in the art, may also be used consistent with theinvention.

Controller 30 is preferably implemented in a microprocessor ormicrocontroller actuated system, which would include all necessaryattendant support circuitry, including clocking, power supplies,volatile and non-volatile memory, display drivers, etc. The power forthe controller is preferably provided by the lift truck battery,although a separate power source may be used in the alternative.

Several advantages are attained through the use of the preferred scaleand dual sensor load cells used therewith. For example, the preferreddual sensor load cells consistent with the invention have the advantageof isolating, measuring and rejecting out residual rejected effects fromthe load cell output, thereby offering greater accuracy and resistanceto non-load force effects. This may enable less precise mechanicalcomponents to be used, may enable lower manufacturing tolerances on loadcell bodies and strain gauges to be used, and may enable less precisestrain gauge matching and placement, while still obtaining acceptablelevels of accuracy. Therefore, lower cost load cells may be constructedhaving equal or greater accuracy than conventional load cell designs. Ofcourse, should the same tolerances and manufacturing precision be usedto construct preferred load cells as in prior designs, the accuracy ofthe new cells may be substantially greater.

Load cells consistent with the invention may also enable less complexand expensive mounting hardware to be used without severely reducing theaccuracy of the overall system. For example, some applications such astruck scales (which measure the weight of over-the-road vehicles, forexample) conventionally require a weighing platform to be coupled tomultiple load cells through flexible attachments such as chains. Thiswas done to center the load on the load cells and minimize any endeffects applied thereto. Through the use of the load cells consistentwith the invention, the end effects would be rejected electrically,rather than using such flexible coupling.

Accordingly, it will be appreciated that various modifications may bemade to the preferred load cells, including different body designs,different transducers, different transducer placements, etc., withoutdeparting from the spirit and scope of the invention. For example, eventwo or more non-load sensors may be utilized to reject multiple yetdisparate non-load effects from the output of the load cells. In thepreferred circuitry for the load cells, this could be accomplished byplacing multiple bridge circuits in parallel with the primary forcesensor bridge. Other modifications will be apparent to one of skill inthe art.

The preferred scales consistent with the invention also provide severaladvantages over conventional designs. For example, the preferred scalesexhibit frictional effect rejection which is useful in many applicationswhere two load cells are coupled in parallel through a non-perfect loadreceiving member and are spaced apart in the direction of the primaryforce vector. By scaling the sensitivities of the parallel load cells inthe manner disclosed herein, any frictional effects in the mounting,such as due to slippage in the mounts which transfers load from one cellto another, will be cancelled out on the summed outputs of the loadcells. Such frictional effects, primarily creep and hysteresis, maytherefore be effectively rejected out in the manner disclosed herein.

One potential application of this feature would be a dual load cellsystem where the dual load cells provide increased capacity, yet betteroverall rejection of frictional effects than either load cell alone. Theload cells used in such a system would be able to be constructed withlower tolerances and precision, and nonetheless obtain acceptableresults from the electrical rejection of these inherent effects in thesystem.

In addition, the cornering correction provided by preferred scales isuseful in any application where a load may be located at differentpoints offset from the loading point for the load cells, particularlywhen it would be difficult or burdensome for an operator to preciselylocate objects at a single point on the scale. The cornering correctioncapabilities of the invention are capable of rendering such scalessubstantially insensitive to load position, thereby rendering the scaleseasier to operate, and less intrusive on the daily routine of anoperator.

It is therefore anticipated that the principles of the invention may beapplied to various scale designs having multiple load cells in whichfrictional effects and/or positional sensitivity is conventionallyexhibited by the scales. Accordingly, numerous modifications and changesto the preferred embodiments will be appreciated by one skilled in theart.

As various additional changes and modifications may be made to thepreferred embodiments without departing from the spirit and scope of theinvention, the invention therefore lies in the claims hereinafterappended.

What is claimed is:
 1. A scale, comprising:a base; a force receivingmember adapted to receive a force applied along a force vector; firstand second load cells, coupled between the force receiving member andthe base in parallel and spaced apart on the force receiving membergenerally in the direction of the force vector such that the load cellsshare the force, each load cell providing an output representative of atleast a portion of the force applied along the force vector; and loadcell driving means comprising means for providing a force signalrepresentative of the force applied to the force receiving member bysumming the outputs of the load cells and frictional effect rejectingmeans for adjusting the relative sensitivities of the outputs of a pairof coupled load cells to reject frictional effects in the scale.
 2. Thescale of claim 1, wherein the force vector is in the direction ofgravity.
 3. The scale of claim 1, wherein each load cell includes asensor for generating the output, the sensor including an input forreceiving an excitation voltage, wherein each sensor comprises aresistive network which varies in resistance in response to the forceapplied along the force vector, and wherein the frictional effectrejecting means includes input scaling means for scaling the relativemagnitudes of the inputs to the sensors.
 4. The scale of claim 3,wherein the input scaling means includes a potentiometer coupled betweena common excitation signal and the inputs of the sensors so thatfrictional effects are rejected by adjusting the potentiometer.
 5. Thescale of claim 3, wherein the sensor of each load cell includes fourstrain gauges electrically coupled in a wheatstone bridge circuit, twoof the strain gauges being oriented on the load cell to be placed intension in response to a force applied along the force vector, and twoof the strain gauges being oriented on the load cell to be placed incompression in response to a force applied along the force vector.
 6. Amethod of weighing an object on a scale including first and second loadcells coupled between a force receiving member and a base in paralleland spaced apart on the force receiving member generally in thedirection of an applied force vector such that the load cells share anapplied force, the method comprising the steps of:generating at eachload cell an output signal representative of at least a portion of theforce applied along the applied force vector; adjusting the relativesensitivities of the output signals of the load cells to rejectfrictional effects in the scale and generate adjusted output signals;and generating a force signal representative of the force applied to theforce receiving member by summing the adjusted output signals of a pairof coupled load cells.
 7. A scale, comprising:a base; a force receivingmember adapted to receive a force applied along a force vector; firstand second load cells, coupled between the force receiving member andthe base in parallel and spaced apart on the force receiving membergenerally in the direction of the force vector such that the load cellsshare the force, each load cell providing an output representative of atleast a portion of the force applied along the force vector; means foradjusting the relative sensitivities of the outputs of a pair of coupledload cells to reject frictional effects in the scale and generateadjusted output signals; and load cell driving means for generating aforce signal representative of the force applied to the force receivingmember by summing the adjusted output signals of the load cells.